A low-pressure glow discharge is a well known phenomenon in physics. Consider an evacuated containing a low-pressure ionizable gas and two electrodes, a positive anode and a negative cathode. When a DC potential is applied between the electrodes current will flow. For small potentials the current is small and occurs in random bursts. If the potential is made sufficiently large a continuous current will flow and, if the current is allowed to increase, light will be emitted, giving rise to the name glow discharge.
Initially electrons and positive ions are accelerated by the applied field toward the anode and cathode respectively, resulting in a flow of current. If the potential is sufficiently large, the electrons can undergo "collisions" with neutral atoms, giving them enough energy for excitation (with subsequent release of that energy as visible light) or ionization (with release of an ion and a second electron). The ions can acquire enough energy from the field to deliver energy in the form of heat to the cathode or to eject one or more electrons from the cathode surface, and these in turn will be accelerated toward the anode. When this occurs, a self-sustaining glow discharge has been formed.
Chemically inert or active gases, or a combination of the two types can be used to produce a glow discharge. Inert gases result in surface heating only. Active gases such as nitrogen, result in both heating and chemical change at the cathode surface. If the gas used includes nitrogen, the cathode will be bombarded by nitrogen ions. This is the physical basis for the ion nitriding form of the glow discharge process. Other similar processes such as carburizing, carbonitriding or siliciding also may be performed in a glow discharge by the choice of appropriate gas mixture.
Ion nitriding is used for case hardening metal parts that are subject to heavy wear. The momentum of the ions striking the workpiece produces the necessary heat and cleaning action on the surface and finally some of the ions will diffuse into the surface. This forms a chemical reaction with the metal resulting in a nitride compound which is very hard and durable. The"case" depth is a function of the amount of time that ions are permitted to bombard the surface under the influence of appropriately selected parameters, i.e., gas mixture, current flow, temperature, pressure and the metal used in the workpiece.
A major diffuculty experienced in the past has been in getting a stable glow established. When the workpiece is first introduced into the vacuum system as a cathode, there are considerable amounts of trapped gases, moisture, oxides and other and other contaminates on the workpiece surface. As the workpiece is heated the contaminates are released into the glow, where they are ionized and, in a local area, may create very high current density tubes. These high current density tubes are often referred to as incipient arcs or streamers. If a tube becomes sufficiently large or intense it may flip over and become an arc. Thereafter the entire glow may concentrate in the low resistance area of the arc, which becomes even more intense and can result in damage to the workpiece.
Control of the streamer or incipient arcing situation has long been a concern in ion nitriding technology, and several approaches have been implemented without complete success. One of the earliest techniques simply involved placing a resistance element in series with the workpiece. When the discharge is stable and uniform with no evidence of streamer action, a fixed current will flow under the influence of steady voltage from the power supply (normally AC). The presence of streamer action tends to cause flow which, in turn, causes a corresponding voltage drop in the series resistance. The net result is that the voltage available for the glow discharge is lower. The lower available voltage tends to extinguish the arc. This is a self-limiting situation. While this approach can be very effective for controlling the problem of arcing it has a gross disadvantage of large amount of power loss in the resistance, often in the order of 50% of the total power consumption. Frequently this technique is coupled with a visual determination by the operator of streamer action and a manual reduction in power by the operator when he feels uncomfortable with what he sees.
Another method suggested in a prior art is to add a series inductor in the primary side of a step up transformer used to provide the high voltage needed to ionize the gas. The inductor, of course, opposes changes in current flow associated with an arc in a manner similar to the resistor, but with less resistive power loss.
Still another method which has been suggested is the use of large banks of capacitors and associated switches on the power supply output to serve as a "dumping place" for the current flow, rather than letting it pass through the gas, when streamer action is detected.
There may be large variations in streamer activity, only some of which are indicative of a situation close to destructive arcing. In each of the prior art approaches the detection of streamer activity is followed by some form of immediate action. There is no processing of the quantity or quality of the streamer action as a basis for determining what remedial action to take.
A typical current density versus voltage characteristics of a glow discharge includes a normal glow discharge region in which the glow covers only a portion of the workpiece and in which a very small change in voltage results in a substantial change in the current density, as the glow expands to cover more of the workpiece surface. The normal glow discharge region is followed by another region called the abnormal glow discharge region in which a substantial increase in voltage is required to increase the current density, as the glow has completely covered the entire workpiece. The glow discharge treatment takes place in the abnormal glow region. In the prior art, entry of the discharge into the abnormal glow region has been determined by visual observation, i.e., when the operator felt that the glow was uniformaly distributed across the workpiece.
In the prior art, control of the workpiece temperature (including increasing the temperature of the workpiece to the desired operating temperature) has been by manual adjustment of the apparatus by the operator in response to a temperature guage reading and his impression of the condition of the glow discharge.
It is thus desirable to provide an integrated automatic control system and method for providing complete control of the operation of an ion nitriding apparatus.